fixed #656

effects/fields/stringfield.cpp

@@ -10,7 +10,8 @@ StringField::StringField(EffectRow* parent, const QString& id, bool rich_text) :
   EffectField(parent, id, EFFECT_FIELD_STRING),
   rich_text_(rich_text)
 {
-
+  // Set default value to an empty string
+  SetValueAt(0, "");
 }
 
 QString StringField::GetStringAt(double timecode)

effects/internal/timecodeeffect.cpp

@@ -83,7 +83,7 @@ TimecodeEffect::TimecodeEffect(Clip* c, const EffectMeta* em) :
   offset_y_val = new DoubleField(offset_row, "offsety");
 
   EffectRow* prepent_text_row = new EffectRow(this, tr("Prepend"));
-  prepend_text = new StringField(prepent_text_row, "prepend");
+  prepend_text = new StringField(prepent_text_row, "prepend", false);
   prepend_text->SetColumnSpan(2);
 }